The secrets to using a magnesium for tinder is edges or fine powder.

"Chunks" don't catch sparks well. When you "shave" a block with a sharp edge (e.g. knife), you tend to get fine edges which can catch a spark. Alternatively, you need very fine powder.

Not my favorite tinder....

As others here have noted, using some duct tape to catch and hold the shavings works better. And, as noted, you better have some good secondary tinder to be lit by the flash fire from the magnesium. Finally, however much you think you need, shave off 3X more.